Additive manufacturing of laser sintered polyamide optically translucent parts

Date
2013-12
Authors
Yuan, Mengqi, 1989-
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Abstract

Lithophane is a translucent image created by varying the plate thickness; the image is observed using a back lit light source. Software Bmp2CnC linearly converts the black and white image grayscale into the thickness, thus generates CAD file and lithophane is fabricated by additive manufacturing machines. Additive manufacturing makes highly complex lithophane fabrication possible. It is a convenient, rapid, green, design-driven, and high precision way to make lithophanes, and no post processing is needed. Optical properties of laser sintered polyamide 12 translucent additive manufactured parts were analyzed in this dissertation. First, selected optical properties of laser sintered polyamide 12 blank plates under different monochromatic light and white light were investigated and applied in production of laser sintered lithophanes to achieve better performance. A spectrophotometer was used to measure the transmittance of visible light through laser sintered polyamide 12 plates as a function of plate thickness. The transmittance decreased with increasing plate thickness according to a modified Beer-Lambert Law, and it varied significantly depending on the monochromatic wavelength. Monochromatic LEDs were used to assess the wavelength dependence on the transmission and contrast. Highest transmission was observed with green light (540 nm), and poorest transmission was measured for yellow light (560 nm). Second, several parameters affecting lithophane manufacturing performance were analyzed including lithophane orientation with respect to light source, brightness and contrast versus plate thickness and grayscale level, quantized plate thickness correction, surface finish quality, and manufacturing orientation. It was found that brightness was relative to the plate thickness. The contrast was defined by the lithophane grayscale level, which was influenced by sintering layer thickness, plate thickness, and sintering orientation. Thinner sintering layers resulted in more grayscale levels of the image and smaller difference between the theoretical thickness and actual thickness. Relatively larger plate thickness defined greater contrast; however, the plate thickness was limited due to the light transmission. Lithophane quality was largely improved by changing the manufacturing orientation from the XY plane orientation to the ZX/ZY plane orientation. The grayscale level changed continuously when parts were constructed in the z orientation. Third, other thermoplastic semi-crystalline materials were analyzed for LS optically translucent part production. Last, plates and lithophanes were built using a different AM platform: stereolithography (SL) with Somos® ProtoGen[Trademark] O- XT 18420 white resin. Different optical properties and lithophane performance were found and compared with PA 12 parts. In conclusion, laser sintered polyamide 12 optical properties varied with light wavelength and reached the maximum under green light. When building in the XY plane, thinner layer thickness (0.07 mm) and relative thicker maximum plate thickness (3.81 mm) leaded to higher contrast and greyscale level. Lithophane quality was largely improved when fabricated in the ZX/ZY plane orientation. Lithophanes made from stereolithography were analyzed but showed lower contrast due to the optical property difference of the white resin. Laser sintered lithophanes serve as an interesting and complex LS industrial application. Optical properties, manufacturing aspects, and other related issues were analyzed and discussed in this dissertation. Future work may include the use of nanocomposites for optimal lithophane performance, and more precise manufacturing processing to improve the lithophane resolution.

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